The correct option is C Effective nuclear charge
The shielding effect due to the f electrons is very low because of the diffused shape of their orbitals. With an increase in the atomic number, the effective nuclear charge (Zeff) experienced by the valence electrons increases along the period. This increase in Zeff causes the radii of atoms or ions to shrink as you go from La to Lu. This is what we call lanthanide contraction.